VisualDOE FAQ

Contents

PROJECT (click to see questions)

BLOCKS (click to see questions)

FACADES (click to see questions)

SYSTEMS

• Where do I specify reheat?
• Can I model Heat Recovery for Exhaust air (exhaust fan)?
• How do I model Heat Recovery?
• I have a number of packaged VAV air-handling units I am attempting to model. Each unit has a water-cooled condenser section. Visual DOE asks to define the closed cooling tower for each VAV unit. Is this creating x number of cooling towers or does the program know that they are all tied into one?
• How does Visual DOE model the condenser water pumps serving the towers? Chilled water pump information is entered for the tower/chiller configuration - but the condenser pumps are not defined.
• I am modeling an AHU that serves say 10 zones. Six zones are 24-hr spaces and require a fan operating constantly .Four zones are on a regular office schedule, requiring fan operation 7 AM to 6 PM. How does DOE model this if the system is a VAV with reheat boxes?
• How does the minimum flow ratio in the Zone folder affect the outside air quantities?
• Why do you use a negative number for the heating capacity?
• I have the VAV unit heating coils set up with a 65 degree leaving air temp and the terminal units are supposed to take care of the heating requirements beyond that. I tried increasing this in 10-degree increments and this took care of the heating requirements, but the supply airflows were much too high.

ORGANIZERS AND EDITORS (click to see questions)

3D VIEWER (click to see questions)

RUNNING SIMULATIONS (click to see questions)

GENERAL (click to see questions)

GLOSSARY OF TERMS (click to see questions)

Systems

• Where do I specify reheat?

Reheat is specified at the zone level. Select a zone on the zones folder and specify the reheat temperature and source. Make sure the control zone for the system has reheat specified.

• Can I model Heat Recovery for Exhaust air (exhaust fan)?

Heat recovery can be modeled only for return air.

• How do I model Heat Recovery?

You may add heat recovery to a system by checking Heat Recovery in the 'Systems Features' frame of the 'HVAC System Editor'. Heat recovery coils are shown on the system diagram connecting the exhaust air stream with the outside air supply stream. To bring up this form, click on the heat recovery coils. Only one input is needed for heat recovery, the heat exchanger effectiveness. This is the ratio of energy exchanged to that, which would occur if the return air were cooled to the outside air conditions.

• I have a number of packaged VAV air-handling units I am attempting to model. Each unit has a water-cooled condenser section. Visual DOE asks to define the closed cooling tower for each VAV unit. Is this creating x number of cooling towers or does the program know that they are all tied into one?

VisualDOE allows the condenser pump inputs to default in this case. The flow (gpm) is based on 10F delta T. The efficiency of pumps and motors are also defaulted. You can check the SS-P output report for sizing information about the condenser pump.

• How does Visual DOE model the condenser water pumps serving the towers? Chilled water pump information is entered for the tower/chiller configuration - but the condenser pumps are not defined.

For condenser pumps serving central plants (chillers), the information is entered on the chiller form on the "Condenser" tab.

• I am modeling an AHU that serves say 10 zones. Six zones are 24-hr spaces and require a fan operating constantly .Four zones are on a regular office schedule, requiring fan operation 7 AM to 6 PM. How does DOE model this if the system is a VAV with reheat boxes?

The air handler operates on a single fan schedule from the occupancy type selected in the 'HVAC System' form. The thermostat schedule for each zone comes from the occupancy selected on the 'Rooms' folder. Therefore, you can have zones on different thermostat schedules served by the same air handler. In the 7 AM to 6 PM zones, you can set the cooling temperature up to 99 at night, but you would have to set the minimum flow fraction to 0 if you wanted to complete shut off those zones. Otherwise it will operate at minimum flow at night.

Sometimes it is necessary to break up the air handler into two systems so that different fan schedules can be used for each set of zones.

• How does the minimum flow ratio in the Zone folder affect the outside air quantities?

The outside airflow input will take precedence. If you choose a minimum flow lower than the outside air input (either based on cfm/person or other means), then DOE-2 overrides the minimum zone flow so that it does not drop below the ventilation rate.

DOE-2 is not the best tool to use for autosizing. It gives a good approximate figure (depending on the system type), but you must go back and look at the zone temperatures to check for yourself how successfully it autosized the equipment. Also the sizing is based on cooling loads.

System-level heating and cooling equipment sizes are listed in the Systems Summary Report in kBtuh for most system types. Unlike zone loads, system sizes do consider outside air load and both heating and cooling indoor design temperatures, but they are based on the non-coincident peak load, i.e. the sum of the zone peaks and not on the building peak load (coincident peak load). These sizes also account for fan heat. System sizes are adjusted to ARI conditions (ARI conditions are different for air-cooled DX equipment than for other types of equipment.) System sizes also include any oversizing ratios you may have specified.

If you have only one system in your building and the fan is VAV (minimum flow ratio < 1) then you can override the default and set SIZING-OPTION = COINCIDENT in the DOE-2 input file (see Editing DOE2.1E files outside of VisualDOE). This will give the coincident system size, including fan heat.

• Why do you use a negative number for the heating capacity?

DOE2 uses negative inputs for heating, that's why it also appears so in the VisualDOE. For input, you can enter either negative or positive, and VisualDOE will convert it to negative for the BDL file.

• I have the VAV unit heating coils set up with a 65 degree leaving air temp and the terminal units are supposed to take care of the heating requirements beyond that. I tried increasing this in 10-degree increments and this took care of the heating requirements, but the supply airflows were much too high.

There are a number of potential causes for underheated zones. These are a few of them:

1. In a VAV system, the minimum flow ratio (ZONE|AIR) may be too low. If the thermostat type is "proportional" then the flow will be at its minimum point in heating mode. This minimum flow may not be high enough to meet the heating loads of the zone. The solution is to increase the minimum ratio (leading to higher energy usage as well) or to specify a "reverse action" thermostat. The reverse action thermostat increases the zone airflow as the heating load increases (just like it does in cooling mode).
2. Another problem, especially in cold climates and heating dominated buildings, is that the total supply air flow is too low.DOE2 sizes the total supply air based on peak cooling loads. The solution is to enter the supply airflow in the HVAC system editor rather than allowing DOE2 to autosize the flow.
3. Zone reheat may be missing. In a multiple-zone system some form of reheat is generally necessary to control space temperature in zones that do not need much heating. Make sure that reheat (or baseboard heating) is specified on the ZONE tab.